(1) Field of the Invention
The present invention relates to a heater unit suitably usable for purposes such as purification of exhaust gas emitted from automobile engine, and the like.
(2) Description of Related Art
Porous ceramic honeycomb structures are known for use as a catalyst, a catalyst carrier or the like for purification of the nitrogen oxides (NOx), carbon monoxide (CO) and hydrocarbons (HC) contained in the exhaust gases emitted from internal combustion engines of automobiles, etc. In recent years, metallic honeycomb structures came to draw attention for the same purpose. Meanwhile, as the regulation of exhaust gas became stricter, it is desired to develop a heater or the like for reduction in exhaust gas during the cold start of engine.
Under the situation, a honeycomb heater comprising a metallic honeycomb structure and electrodes attached thereto for electrical heating of the structure was developed. Ordinarily, a honeycomb heater is provided in the exhaust gas flow path of automobile or the like, in the form of a heater unit comprising a metallic casing and said honeycomb heater held in the casing via an appropriate holding member.
As such a heater unit, the present applicant proposed, in Japanese Patent Application Kokai (Laid-Open) No. 259543/1995, a heater unit comprising a metallic casing and an electrically heatable honeycomb heater held in the metallic casing via a metallic holding member, wherein at least either of the boundary between the honeycomb heater and the holding member and the boundary between the holding member and the casing has been subjected to an insulation treatment and the holding member can absorb the displacement of the honeycomb heater taking place in a direction vertical to the flow direction of exhaust gas and can fix the honeycomb heater against the flow of exhaust gas.
The present applicant also proposed, in Japanese Patent Application Kokai (Laid-Open) No. 4521/1996, an improvement in the heater unit disclosed in Japanese Patent Application Kokai (Laid-Open) No. 259543/1995, that is, a flow-controlling means for keeping the by-pass flow of exhaust gas at 20% or less.
The heater unit comprising a casing and a honeycomb heater held in the casing, used in an automobile or the like must firstly have a structure for holding the honeycomb heater so that the honeycomb heater is not damaged even under severe operating conditions such as vibration, thermal impact and the like. Further in the heater unit, it is desired that the amount of the unpurified exhaust gas passing through the gap between the casing and the honeycomb heater, i.e. the by-pass flow is minimized.
With respect to the reduction in the by-pass flow, it is described, in the above-mentioned Japanese Patent Application Kokai (Laid-Open) No. 259543/1995, to provide a ceramic mat on the outer periphery (side) of a honeycomb heater for prevention of the by-pass flow of exhaust gas. However, this ceramic mat has a fear that it is worn owing to the vibration of the metallic holding member under actual use conditions, inviting gradual increase in by-pass flow.
Also, in the above-mentioned Japanese Patent Application Kokai (Laid-Open) No. 4521/1996, it is disclosed to suppress the by-pass flow of exhaust gas to 20% or less, specifically by constructing the casing so that the distance between the casing and the outer surface of the honeycomb heater became 3 mm or less at the gas inlet and/or the side of the honeycomb heater. However, making small the distance between the casing and the honeycomb heater has a limitation because of a fear of short-circuiting between the casing and the honeycomb heater.
In view of the above situation, the present invention aims at providing a heater unit comprising a casing and a honeycomb heater held in the casing, wherein the honeycomb heater is not damaged even under severe use conditions and has sufficient durability and wherein the by-pass flow of exhaust gas is prevented and the whole exhaust gas can pass through the honeycomb heater.
According to the present invention, there is provided a heater unit comprising:
a metallic casing, and
a honeycomb heater held in the casing via a metallic holding member, comprising a metallic honeycomb structure having a large number of parallel passages extending in the direction of the flow of an exhaust gas passing through the heater unit and at least one electrode for electrical heating of the honeycomb structure, attached to the honeycomb structure,
in which heater unit the holding member has such a structure as (1) is fitted to part of the outer surface of the honeycomb heater to allow the honeycomb heater to have a stable shape and (2) substantially blocks the flow path of exhaust gas between the honeycomb heater and the casing.
In the heater unit of the present invention, it is preferred that the holding member is fitted to the outer periphery (side) and/or the periphery of face, of the honey-comb heater. It is also preferred that the holding member and the honeycomb heater are insulated from each other via an insulating member. It is also preferred that the holding member is buried in a groove formed at the outer periphery (side) and/or the periphery of face, of the honeycomb heater, and fixed. It is also preferred that the holding member consists of at least one ring. The holding member may consists of a plurality of divided parts. It is also preferred that the holding member substantially blocks the flow path of exhaust path at the peripheral portion of the honeycomb heater which is not heated, or at the portion at the portion of the honeycomb heater which is inferior in heat-generating property to the central portion of the honeycomb heater.